The human and financial costs of low back and neck disorders to our society are overwhelming. The causes of most of these disorders are unknown, but biomechanical factors are clearly involved. Research is proposed to continue to examine mechanical functioning of the human trunk and neck. Some emphasis will be given to pathological functioning, but major emphasis will be placed on normal functioning. To understand the mechanics of spine pathologies, a good understanding of healthy spine mechanics seems required. Quasic-static biomechanical models of the cervical region, and dynamic models of the cervical and lumbar regions will be constructed to predict muscle contraction forces and spine motion segment reactions required for physical task performances. Model prediction validity will be tested in-vivo through quantitative myoelectric measurements. Slow motions, fast single motions and fact cyclic motions will be studied. Spine manipulation biomechanics will be explored, both experimentally and analytically. Whole body vibration response and pathological intervertebral disc/posterior element load sharing will be analyzed. Muscle relaxation phenomena will be measured experimentally, and heavy lifts-with-twists examined both experimentally and analytically.